Overpressure retorts are used for the in-container preservation of foodstuffs, either for pasteurization or sterilization processes. In general, these machines use a combination of pressure and temperature to sterilize packaged food according to a predefined schedule. Their popularity has increased in the past few years because of the development of processes that allow for the use of media other than only saturated steam. Using other fluids enables the application of an additional partial overpressure on top of the basic steam vapor pressure associated with the process temperature. Such additional overpressure is useful in coping with new types of containers that are being introduced on the market.
The additional overpressure is usually achieved by adding air to the interior of the retort vessel. As air has poor heat transfer properties, the mixture of steam and air has to be assisted for good heat transfer to the containers and their content. This can be accomplished in any number of ways. In steam-air retorts, the mixture is recirculated through the retort load by means of fans. In full immersion retorts, the load is immersed in water. In trickling water type retorts, water is trickled from the top of the retort to the bottom, passing through the load in-between. In spray water retorts, water is sprayed from the top (and optionally also from the sides) of the container through the load. The latter type can make use of external means to heat the water or alternatively can have direct steam injection into the vessel.
FIG. 1 illustrates a recirculation system in a known spray water retort. A cylindrical pressure vessel 10 houses a load 12, a header 14, and various spray nozzles 16. The load is supported on a horizontal pallet (not shown). Distribution pipes 18 extend the longitudinal length of the vessel. Using the various spray nozzles 16, the distribution pipes 18 direct process water into the vessel and onto the load. The header 14 is positioned at select locations needed to support the distribution network (e.g., in the middle of the vessel). Use of side spray nozzles 20 is optional, depending on the type of load being processed. In general, the spray pattern shown provides a good distribution of process water through the load. The process water is collected in a lower sump 22, passed through a strainer 24, and recirculated through the system via a recirculation pump 26. The recycled process water is passed through piping 28 and one or more filters 30 and then reintroduced to the distribution pipes. Control valves 32 regulate the recirculation process.
Referring to FIG. 2, the nozzles used have a solid cone spray characteristic, with a cone angle β of approximately 75 degrees. This angle can vary slightly, depending on the pressure of the fluid in the distribution pipes. In one arrangement, this pressure is on the order of 1 bar overpressure. The distance D between the nozzles and the load in prior art arrangements is generally in the range of about 70 mm to about 200 mm. The distance is chosen so that the spray water flowing through the load provides a good temperature distribution. The load may be baskets (which are typically used for cans and jars) or stacks of trays (which are typically used for containers that are not rigid enough to allow stacking on top of each other, e.g., plastic lunch bowls and pouches).
During use, the load is subjected to a preservation phase having a predefined temperature schedule and a predefined pressure schedule. The temperature distribution is important within each individual basket or stack, and also between the different baskets or stacks placed along the length of the retort. The fluid flow per nozzle and the number of nozzles in the retort system determine the total flow of process water being continuously recirculated over and through the load. From experience it was found that satisfactory temperature distributions are achieved with a flow capacity per processing position (i.e., a basket or stack) of approximately 30 m3/hr per cubic meter load for static processing retorts, and, 40 m3/hr for agitation processing retorts. A typical flow per nozzle is in the order of 17 liters/min. In some arrangements, the recirculation system is also used for cooling the load after completion of the preservation phase.
Recently, a new type of paperboard package has been introduced for use with various types of foodstuffs. These packages are generally composed of a coated paperboard folded into a rectilinear shape. Currently, fluids such as juices, soups, soy milk, etc. are packaged in these kinds of containers. The packages have some degree of protective coatings on their surfaces, but, in general, are susceptible to fluid absorption along their exposed edges. The edges typically do not receive coatings due to manufacturing cost considerations. The amount of absorption that may occur is a factor in determining the package quality. If there is too much absorption, the package will be considered defective.
The spray water retort process described above has been found to cause excessive fluid absorption in some paperboard packages. Thus, a need exists for an improved retort system that reduces the amount of absorption and thereby reduces the number of defective packages. The present invention is directed to fulfilling these needs and others as described below.